If you need to analyse the values of a picture or a painting, you can use this heatmap inside krita.
Filters > Map > Gradient Map > XML
Considering your base picture is rather ok, not too exposed and not too much flash.
This heatmap and the 7 values black and white gradient should be enough to get yourself started with skintones and light analysis.
Courtesy of Anna Wakitsch for providing a grid for Photoshop, I just made it quickly myself for Krita.